How to remove spark plug insulator ?

[Larry DeCamp]

I am fabricating a plunger for dial indicating accurate TDC to set Pmag timing. It would be simple to sleeve a spark plug and insert a round tip or roller tip push rod to sense the piston. Does anyone know how to get the insulator out of the steel spark plug body ?

[Jrskygod]

Hammer and punch worked for me!!!!!

[Canadian_JOY]

Wow, this seems like the hard way to accomplish this task. I use a brand new traditional yellow wooden pencil with a rubber eraser on the end. I know somebody will say the rubber eraser may come off - that's why I use a new one that hasn't seen any in-service abuse. I keep this fancy and expensive tool in the same box as my compression tester and mag timer. The same pencil has been doing compression checks on several aircraft for at least the last decade and it doesn't even have a scratch on it.

And yes, you really can feel when the piston is at TDC if you are paying attention to the movement of the pencil.

[Tbone]

I believe he is referencing finding the true TDC. You can push the center out with a light application of a press. Just push center electrode out from the piston side.

[Canadian_JOY]

"True TDC"? Educate me, please! I always thought TDC was when the piston is at the top of its stroke, easily sensed by feel to within a fairly small margin of error of crank rotation. What am I missing?

If one really wants to use a dial indicator, what would be wrong with using the compression tester spark plug adapter - it's already hollow, and, with the air fitting attached, provides a good guide for a pushrod.

[lr172]

Originally Posted by Canadian_JOY

"True TDC"? Educate me, please! I always thought TDC was when the piston is at the top of its stroke, easily sensed by feel to within a fairly small margin of error of crank rotation. What am I missing?

If one really wants to use a dial indicator, what would be wrong with using the compression tester spark plug adapter - it's already hollow, and, with the air fitting attached, provides a good guide for a pushrod.

Due to the geometry of the crank/rod rotation, finding TDC requires precision. There is very little piston movement around 10* either side of TDC. The difference between TDC and 5* either side is probably only .010" of piston travel.

Precision is necessary for determining your actual TDC for things like validating the flywheel mark, setting cam or ignition timing without a trusted mark, etc. This level of precision is not necessary for rough TDC measurement, like for compression tests.

Larry

[lr172]

Originally Posted by Larry DeCamp

I am fabricating a plunger for dial indicating accurate TDC to set Pmag timing. It would be simple to sleeve a spark plug and insert a round tip or roller tip push rod to sense the piston. Does anyone know how to get the insulator out of the steel spark plug body ?

It might be easier to fabricate a piston stop from an 18mm bolt. When using a piston stop, you put a degree wheel on your flywheel (can use sharpies). The piston stop let's you find common points on both sides of TDC (TDC sits directly in the middle of those two points). Even with a dial indicator, this method will give you more accurate results and is the standard for finding TDC.

Larry

[Tbone]

I was nearly ready to type the same thing. Finding the true TDC is pretty important when timing the magnetos. Some engines have no built in marks to verify off of. And I don't trust them either. (Well most of the time!)

[bobnoffs]

i KNOW trying to drill it out doesn't work.

[Larry DeCamp]

Thanks to all for the input. The insulator does come out from the plug wire side, however, there are snap rings and undercuts that are designed to secure the ceramic. So, plan a lot of pounding with punces to fracture and remove pieces.

The piston stop bolt is clearly the best approach and the degree scale on the starter can just be masking tape to mark 3 places. The inconvenience is a stable pointer to reference, but I think it is worth it .